1. Field of the Invention
This invention relates to an optical system for X rays and a multilayer mirror used therefor.
2. Description of Related Art
The recent development of technology for applying X rays to an object and obtaining an image of the object by the X rays transmitted through the object has been applied for industrial tests, biological analysis, etc. Particularly in the fields where microscopic images by X rays are necessary, there is a great demand for producing a microscopic image of a specific chemical element by transmitted X rays and X-ray luminescence from a specimen.
The reason why a transmission image of only a specific element can be obtained by X rays is that the absorption coefficients of elements have different wavelength dependence in the X-ray wavelength range. FIG. 1 shows how protein, nucleic acid and water absorb X rays, with wavelengths and absorption coefficients represented on the horizontal and vertical axes, respectively. In the figure, the O, N and C edges are the absorption edges of X rays by oxygen, nitrogen and carbon, respectively. As is apparent from the figure, each element has its specific discontinuous absorption edge: for example, carbon has a discontinuous absorption edge at about 44.ANG., and nitrogen at about 31.ANG.. Therefore, when a transmission image of protein is obtained by X rays of a wavelength in the neighborhood of the absorption edge where the absorption coefficient is maximum, for example, X rays of a wavelength of about 40.ANG., one can expect a high-contrast good image showing the distribution of carbon in protein. Furthermore, if one makes use of the difference between a transmission image obtained by X rays of a wavelength slightly shorter than that of the absorption edge of an element to be detected, at which wavelength the absorption coefficient is high, and a transmission image obtained by X rays of a wavelength slightly longer than that of the absorption edge, at which wavelength the absorption coefficient is low, then one can obtain a better image even if the X rays are weak. This is because the use of the difference can eliminate unnecessary signals resulting from the absorption by elements other than the element to be detected. Such a method is achieving success in medical radiography using sychrotron radiation and the like fields.
On the other hand, owing to the recent progress of manufacturing technology, various optical devices for X rays have been developed for commercial use and applied for the imaging optical systems of X-ray microscopes. In particular, attention is recently drawn to a Schwarzschild optical system comprising a concave mirror and a convex mirror each having a multilayer formed on a substrate since it has a better imaging performance than other X-ray optical systems (for example, Walter type optical system). It has been proposed to apply this optical system for the imaging optical system of a microscope or an exposing apparatus for semiconductor manufacturing in which a high imaging performance is required. If the above-mentioned method can be also applied to the Schwarzschild optical system, an attractive microscope system can be realized.
A Schwarzschild optical system as an optical system for X rays, the perspective view of which is shown in FIG. 2, is a focusing optical system in which a concave mirror 1 having a circular opening in its center and a convex mirror 2 are arranged coaxially with their reflecting surfaces opposed to each other and which focuses X rays radiated from an object point O at an image point I through the opening of the concave mirror 1 via the two reflecting mirrors. In order to reflect the X rays radiated from the object point O with a high reflectance to focus them at the image point I, each of the two spherical mirrors 1 and 2 comprises a substrate and a uniform multilayer formed on the substrate by laying alternately two kinds of substances whose refractive indexes are much different from each other.
The multilayer has such a structure as shown in FIG. 3: a layer of substance a having a thickness of d.sub.1 and a layer of substance b having a thickness of d.sub.2 are laid on a substrate 3 alternately and repeatedly with a periodic thickness of d. With this multilayer, a high reflectance can be obtained under Bragg's condition, that is, EQU m.lambda.=2d sin .omega.
where .lambda. is the wavelength of X rays, d is the periodic thickness of the multilayer, .omega. is the grazing angle of X rays with respect to the multilayer, and m is an integer. Especially, X rays of a wavelength defined by the following formula are efficiently reflected: EQU .lambda.=2d sin .omega. (that is, m=1).
Additionally, in FIG. 3, numerals 4 and 5 represent incident X rays and reflected X rays, respectively, and 0 denotes the incident angle.
Thus, in FIG. 2, only when X rays having a specific wavelength are incident on the spherical mirrors 1 and 2 at a specific incident angle, they are efficiently reflected. Particularly in the Schwarzschild optical system, since the grazing angle .omega. is about 90.degree. , only the X rays of a wavelength of .lambda.=2d are focused. So, it is very difficult to focus X rays of two different wavelengths at the same time. Therefore, although it has been also expected in the Schwarzschild optical system to use the difference between the transmission images obtained by X rays of two wavelengths which are on both sides of the absorption edge of an element, the application of this method has been difficult because of the above reason.